Calls to error () can cause SIGTTOU to send gdb to the background.
For example, on an Arm build:
(gdb) b main
Breakpoint 1 at 0x10774: file /build/gdb/testsuite/../../../src/binutils-gdb/gdb/testsuite/gdb.base/watchpoint.c, line 174.
Starting program: /build/gdb/testsuite/outputs/gdb.base/watchpoint/watchpoint
+ Stopped ../gdb ./outputs/gdb.base/watchpoint/watchpoint
Cannot parse expression `.L1199 4@r4'.
warning: Probes-based dynamic linker interface failed.
Reverting to original interface.
*** This bug has been marked as a duplicate of bug 25010 ***